Bottom Line:
Thereby, we revealed a critical role for Bim and Bmf as regulators of HSPC dynamics both during early engraftment and long-term reconstitution.HSPCs derived from wild-type donors were readily displaced by Bim- or Bmf-deficient or Bcl-2-overexpressing HSPCs as early as 10 days after engraftment.Finally, we provide proof of principle that RNAi-based reduction of BIM or BMF, or overexpression of BCL-2 in human CD34(+) cord blood cells may be an attractive therapeutic option to increase stem cell survival and transplantation efficacy.

Mentions:
To define the relevance of the early reconstitution advantage of bim−/− or bmf−/− LSK cells for the kinetics of haematopoietic regeneration, we analysed BM, spleen and thymus at 5 and 10 days after reconstitution. At day 5, all haematopoietic organs were reduced in size as well as cellularity and almost no differentiated Ly5.2+ cells were detectable, independent of the donor-genotype (unpublished observation). Similarly, at Day 10 most of the cells were host-derived. At this time point, Ly5.2+ and thus LSK-derived donor cells were detectable up to the DN3-4 stage and a trend to higher percentages of Ly5.2+ cells was already observed in wt:bim−/−, wt:bmf−/− and wt:bcl-2 tg chimeras when compared to wt:wt chimeras. The percentages of splenic Ly5.2+ monocytes and granulocytes were likewise increased in the absence of Bim or Bmf (Fig 4A). In the peripheral blood, significant advantages of bim−/−, bmf−/− and bcl-2 tg cells were first noted 2 and 4 weeks after transplantation and affected both lymphocytes and myeloid cells (Fig 4B). Of note, whereas bmf−/− cells were clearly less potent in replacing wt cells in long-term reconstitution experiments, when compared to bim−/− cells, they appeared equally potent during the initial haematopoietic reconstitution (Fig 4A and B). This observation is in line with our hypothesis that both, increased stem cell survival as well as accumulation of mature lymphatic and myeloid cells, contribute to the phenotypes observed in our long-term assays. Hence, the role of Bmf may be more critical during initial reconstitution events.

Mentions:
To define the relevance of the early reconstitution advantage of bim−/− or bmf−/− LSK cells for the kinetics of haematopoietic regeneration, we analysed BM, spleen and thymus at 5 and 10 days after reconstitution. At day 5, all haematopoietic organs were reduced in size as well as cellularity and almost no differentiated Ly5.2+ cells were detectable, independent of the donor-genotype (unpublished observation). Similarly, at Day 10 most of the cells were host-derived. At this time point, Ly5.2+ and thus LSK-derived donor cells were detectable up to the DN3-4 stage and a trend to higher percentages of Ly5.2+ cells was already observed in wt:bim−/−, wt:bmf−/− and wt:bcl-2 tg chimeras when compared to wt:wt chimeras. The percentages of splenic Ly5.2+ monocytes and granulocytes were likewise increased in the absence of Bim or Bmf (Fig 4A). In the peripheral blood, significant advantages of bim−/−, bmf−/− and bcl-2 tg cells were first noted 2 and 4 weeks after transplantation and affected both lymphocytes and myeloid cells (Fig 4B). Of note, whereas bmf−/− cells were clearly less potent in replacing wt cells in long-term reconstitution experiments, when compared to bim−/− cells, they appeared equally potent during the initial haematopoietic reconstitution (Fig 4A and B). This observation is in line with our hypothesis that both, increased stem cell survival as well as accumulation of mature lymphatic and myeloid cells, contribute to the phenotypes observed in our long-term assays. Hence, the role of Bmf may be more critical during initial reconstitution events.

Bottom Line:
Thereby, we revealed a critical role for Bim and Bmf as regulators of HSPC dynamics both during early engraftment and long-term reconstitution.HSPCs derived from wild-type donors were readily displaced by Bim- or Bmf-deficient or Bcl-2-overexpressing HSPCs as early as 10 days after engraftment.Finally, we provide proof of principle that RNAi-based reduction of BIM or BMF, or overexpression of BCL-2 in human CD34(+) cord blood cells may be an attractive therapeutic option to increase stem cell survival and transplantation efficacy.